1. Field of the Invention
The invention relates to well logging with nuclear tools. In particular, it relates to apparatus and methods for the determination of the thermal neutron capture cross section of the borehole and the formation surrounding the borehole. In addition, the invention also relates to apparatus, and methods for determining formation porosity.
2. Background Art
The characteristics of geological formations are of significant interest in the exploration and production of subsurface water and mineral deposits, such as oil and gas. Many characteristics, such as the hydrocarbon volume, porosity, lithology, reservoir location, and permeability of a formation, may be deduced from certain measurable quantities. Among these quantities are: density, porosity, photoelectric factor (Pe), hydrogen index, salinity, and thermal neutron capture cross section (Sigma). These quantities are typically measured by logging-while-drilling (LWD) or wireline tools.
A typical logging tool carries a source that radiates or emits energy into the formation and one or more detectors that can sense the resulting interactions of the radiation. Detected signal data are typically transmitted uphole, temporarily stored downhole for later processing, or combined in both techniques, to evaluate the geological formation from which the data was gathered.
The determination of the formation capture cross section (Sigma) allows the determination of the oil saturation of the formation, if the salinity of the formation water, the capture cross section of the formation matrix and the formation porosity are known. Sigma may be determined from the decay times of the gamma rays produced following the capture of thermal neutrons by nuclei in the formation. U.S. Pat. No. 3,379,882 issued to Youmans discloses methods for determining formation thermal decay time or pulsed-neutron capture cross section. The method involves irradiating a formation from a borehole tool with a short burst of fast neutrons (pulsed neutrons) and measuring the decline rate of slow neutrons or gamma rays which result from thermal neutron capture in the formation. These measurements provide an indication of the identity of the nuclei of the materials present in the formation.
In its simplest form, a Sigma logging tool consists of a pulsed neutron generator and one gamma-ray detector. The gamma-ray detector uses two or more time gates following the burst to determine the characteristic die-away time of the capture gamma-ray after the end of the burst. The die-away time is inversely related to the apparent capture cross section of the formation as shown in equation (1).
                    ∑                  =                      4550            T                                              (        1        )            
where Σ is the macroscopic formation capture cross section in capture units (c.u.) and τ is the time constant of the time decay in microseconds, which is assumed to be exponential:N=N0·e−t/τ  (2)
The lifetime curve of thermal neutrons is a composite of captures occurring in the borehole including casing and surrounding cement in cased holes, in the porous invaded zone surrounding the borehole, and in the uninvaded formation beyond. All these capture processes occur with different decay times, and it is possible to decipher the formation decay process from the composite capture processes. A typical approach is to monitor the capture process with two or more time windows after the neutron burst. The two or more time window measurements may then be used to derived the desired decay times. A preferred method for making a neutron lifetime measurement, for quantitative determination of formation characteristics, is to observe the complete decline curve of the neutron induced radiation (thermal neutrons or capture gammas) from the termination of the neutron pulse to the disappearance of all induced radiation (excluding the activation or background gammas).
A refinement of the technique consists of using dual bursts, i.e. two bursts of unequal length (duration) and measuring the decay times after each of the bursts. This technique allows an excellent separation between the apparent borehole and the formation decay times. For detailed discussion of this technique, see U.S. Pat. No. 4,721,853 issued to Wraight and assigned to the assignee of the present invention.
Modern Sigma logging tools use at least two gamma-ray detectors at two different axial spacings from the pulsed neutron source. The use of two different spacings makes it possible to correct for environmental effects, which influence the measured (apparent) sigma. Specifically, the detector with the shorter spacing is more susceptible to the capture cross section of the borehole (borehole fluid and if present casing and cement) and also more sensitive to the effect of neutron diffusion (as opposed to neutron capture) on the apparent neutron decay time. Therefore, a comparison between signals detected by the short spacing detector and the long spacing detector can provide a compensation for these effects.
The presence of two detectors also makes it possible to determine count rate ratios. The ratios can be computed either between the total average count rates of the two detectors, between the inelastic count rates or between the capture count rates. These ratios are inversely related to the formation porosity, i.e. the ratio of the short-spacing and long-spacing detector count rates will decrease with increasing formation porosity. Therefore, the ratios can be used to determine the porosity of the formation in a manner similar to the neutron porosity measurement of the CNL tools. The use of a ratio, while reducing some of the environmental effects on the final answer, is largely dictated by the fact that the neutron output of the pulsed generator is not adequately known. In addition, the output of a pulsed neutron generator changes as a function of time, temperature and age of the generator.